Floodgates open

As a flurry of blockbuster drugs lose their 20-year patent protection, the market is splitting wide open as drugmakers offer competing generic versions.

One of the best-known drugs going generic this year is the cholesterol-lowering drug Lipitor (atorvastatin), which brought in an estimated $5.3 billion in U.S. sales in 2010. The cost of Lipitor is expected to drop from an average cost of $228 per prescription to $46 per prescription in its generic form. It is slated to go generic in November.

Zyprexa (olanzapine), Eli Lilly's top-selling drug, prescribed for the treatment of schizophrenia and bipolar disorder, is expected to drop from $747 per prescription to $149, once it goes generic in October.

The following is a partial list of other notable blockbusters that either have already gone off patent or are in the pipeline and expected to do so by the end of 2014: